Signal Arrays do not work with HTML-Elements?

I tried to debug my problem for a while until I realized that this works:

import { useSignal } from "@preact/signals";

export default function Island() {
  const activated = useSignal(false);
  return (
    <button
      onClick={() => {
        activated.value = !activated.value;
      }}
      class={activated.value ? "bg-blue-800" : "bg-red-800"}
    >
      Test
    </button>
  );
}

import { useSignal } from "@preact/signals";

export default function Island() {
  const activated = useSignal(false);
  return (
    <button
      onClick={() => {
        activated.value = !activated.value;
      }}
      class={activated.value ? "bg-blue-800" : "bg-red-800"}
    >
      Test
    </button>
  );
}

but this does not:

import { useSignal } from "@preact/signals";

export default function Island() {
  const activated = useSignal([false]);
  return (
    <button
      onClick={() => {
        activated.value[0] = !activated.value[0];
      }}
      class={activated.value[0] ? "bg-blue-800" : "bg-red-800"}
    >
      Test
    </button>
  );
}

import { useSignal } from "@preact/signals";

export default function Island() {
  const activated = useSignal([false]);
  return (
    <button
      onClick={() => {
        activated.value[0] = !activated.value[0];
      }}
      class={activated.value[0] ? "bg-blue-800" : "bg-red-800"}
    >
      Test
    </button>
  );
}

10 Replies

AwsmBuff•13mo ago

This is just an example but in my project: Being able to use an array at that point instead of hardcoding all the signals would help me a lot. Am I using it wrong?

ioB•13mo ago

You need to make sure to set the actual signal value alternatively, you could have an array of signals

AwsmBuff•13mo ago

At first: Thank you for your help! What do you mean by that? I thought I am setting the signal value with activated.value[0] = !activated.value[0];. At least that is what I am doing in the first example and there it works. Yea, that is what I am now trying to do but it gets ugly in my case and I wondered if an array inside of a signal would work too.

ioB•13mo ago

preact signals don’t do deep observability. You can do what you’re doing right now, but you need to reassign the array afterwards. activated.value = activated.value should work it’s similar to svelte state if you’ve ever done that before

AwsmBuff•13mo ago

Reassign like right after negating the value? (Just asking because I tried that and it did not work.) Doesn't sound familiar to me 😅

ioB•13mo ago

marvin probably knows better than me 😅

marvinh.•13mo ago

yup, like lino said: Signals track reactivity through assignment. That means a signal will only update when you do signal.value = whatever . Pushing into an array or mutating the contents of the array is not tracked. Assigning to an index in the array assigns to the array, not the signal. So it's not tracked. We thought about doing deep tracking like this when creating signals, but ultimately you always end up having to Proxy every collection type in JS like Array, Map, Set,... which gets a bit nasty with edge cases. Add on top that it blurs the line between when something is tracked and when it's not. So we opted for the simpler model. @lino-levan nah you're already spot on. Just providing a little more historic context 😄

AwsmBuff•13mo ago

Thank you very much for the detailed explanation. Would you recommend to do an array of signals instead a signal containing an array then too or is there a better way?

marvinh.•13mo ago

Depends on what parts of the data you want to react to in your UI. If you only ever deal with the full array you can just clone it and assign it to update it.

const next = activated.value.slice(); // clone old array
next[0] = !activated.value[0]; // update value
activated.value = next; // trigger signals update

const next = activated.value.slice(); // clone old array
next[0] = !activated.value[0]; // update value
activated.value = next; // trigger signals update

some devs also may use destructuring to clone arrays. Both work the same way

const next = [...activated.value]

const next = [...activated.value]

AwsmBuff•13mo ago

Cool, thanks again for the suggestions! And @lino-levan for the first aid! Hope this will help others in the future as well.